The invention relates generally to cryogenic fluid delivery systems, and, more particularly, to a cryogenic fluid delivery system that vaporizes a portion of a pumped cryogenic liquid stream and uses the vaporized cryogen to power a linear actuator which, in combination with a supplemental linear actuator, drives the system pump.
Cryogenic fluids, that is, fluids having a boiling point generally below −150° F. at atmospheric pressure, are used in a variety of applications. For example, liquid natural gas (LNG) is an alternative fuel for vehicles that is growing in popularity. As another example, laboratories and industrial plants use nitrogen in both liquid and gas form for various processes.
Cryogenic fluids are typically stored as liquids that require pressurization and sometimes heating prior to usage. The liquid nitrogen stored by laboratories and industrial plants typically must be pressurized prior to use as a gas or liquid. In the case of LNG fueling stations, the LNG is typically dispensed to a vehicle in a saturated state with a pressure head that is sufficient to meet the demands of the vehicle's engine. The saturated state of the LNG prevents the collapse of the pressure head while the vehicle is in motion. Alternatively, the LNG may be stored onboard a vehicle in an unconditioned state. The onboard LNG may then be pressurized and heated as it is provided to the vehicle engine.
A common method of saturating the LNG is to heat it as it is stored in a delivery system storage tank. This is often accomplished by removing a quantity of the LNG from the tank, warming it (often with a heat exchanger) and returning it to the tank. Alternatively, the LNG may be heated to the desired saturation temperature and pressure through the introduction of warmed cryogenic gas into the tank.
Warming LNG in the delivery system tank is undesirable, however, because it reduces the hold time of the tank. The hold time of the tank is the length of time that the tank may hold the LNG without venting to relieve excessive pressure that builds as the LNG warms. Furthermore, refilling a tank when it contains saturated LNG requires specialized equipment and additional fill time. Warmed LNG also is less dense than cold LNG and thus reduces tank storage capacity. While these difficulties may be overcome by providing an interim transfer or conditioning tank, such tanks have to be tailored in dimensions and capacities to specific use conditions. Such use conditions include the amount of fills and pressures expected. As a result, the variety of applications for such a delivery system are limited by the dimensions and capacities of the conditioning tank.
Another approach for saturating the LNG prior to delivery to the vehicle tank is to warm the liquid as it is transferred to the vehicle tank. Such an approach is known in the art as “Saturation on the Fly” and is illustrated in U.S. Pat. No. 5,787,940 to Bonn et al. wherein heating elements are provided to heat LNG as it is dispensed. A disadvantage of the system of the Bonn et al. '940 patent, however, is that electricity is required to operate the heating elements. In addition, the system of the Bonn et al. '940 patent employs a conventional pump and thus suffers from initial system, operating and maintenance cost disadvantages.
U.S. Pat. No. 5,687,776 to Forgash et al. and U.S. Pat. No. 5,771,946 to Kooy et al. also illustrate systems that dispense cryogenic fluid and perform saturation on the fly. The systems disclosed in these two patents use heat exchangers, and therefore ambient temperature, to warm the cryogen as it is transferred to vehicles. The systems, however, also use conventional pumps to dispense the cryogen.
Prior art cryogenic fluid delivery systems typically pressurize and transport the cryogen via pumps that are powered by electricity or-mechanically with fuels such as gas or oil. This significantly increases the operating costs of the delivery system. In addition, many prior art cryogenic fluid delivery systems use pumps that are of the centrifugal or “single-acting” piston variety. Single-acting piston pumps have a single chamber in which an induction stroke of the piston is followed by a discharge stroke. A disadvantage of such pumps is that they have relatively low pump delivery rates which results in increased fueling times.
In answer to the above concerns, some prior art pumps are powered by a “dual-acting” piston that is driven by pressurized gas or liquid. For example, U.S. Pat. No. 3,234,746 to Cope discloses a pump for transporting liquid carbon dioxide from a storage tank. The pump is powered by carbon dioxide vapor from the head space of the storage tank. The pump of the Cope '746 patent features two pistons and corresponding cylinders with a common piston rod. Carbon dioxide vapor is provided to opposing sides of the driving cylinder in an alternating fashion so that the other piston is driven. As a result, the driven piston pumps the liquid carbon dioxide in the tank to a second tank or container. The driven piston is dual-acting so that it pumps the liquid carbon dioxide from both sides of the piston, that is, liquid carbon dioxide is pumped during every stroke of the piston. Carbon dioxide vapor exhaust from the driving cylinder is vented to the atmosphere.
While the pump of the Cope '746 patent is inexpensive to operate, the transfer rate and discharge pressure that it may achieve is limited by the pressure that is available in the head space of the storage tank. In addition, the liquid carbon dioxide in the storage tank must be warmed for the pump to operate. As described previously, warming the liquid carbon dioxide, or any cryogenic liquid, reduces the hold time of the tank. The pump of the Cope '746 patent also fails to provide a means for heating the liquid carbon dioxide as it is transferred.
In response to the limitations in delivery rates of prior art pumps, the pump illustrated in U.S. Pat. No. 5,411,374 to Gram was developed. The Gram '374 patent features a dual-acting piston arrangement that is similar to the pump of the Cope '746 patent. The pump of the Gram '374 patent, however, is powered by a hydraulic motor circuit which provides liquid to opposing sides of the driving piston in an alternating fashion. While the pump of the Gram '374 patent overcomes the discharge pressure shortcomings of the pump of the Cope '746 patent and the prior art, the hydraulic motor circuit provides for significantly increased operating costs.
An additional problem with the pump of the Gram '374 patent, and other pumps that use linear actuators, such as hydraulic cylinders, is that when the discharge pressure of the pump gets high, such as 3000 psi or greater, the size of the actuator becomes very large. Indeed, the size of the actuator may become even larger than the remaining portion of the pump. Such an arrangement is impractical from a production and operation standpoint.
As explained in commonly owned U.S. patent application Ser. No. 10/054,784 to Emmer et al., a low pressure cryogenic liquid may be pumped to a higher pressure and then a portion of the liquid may be vaporized with an ambient air heat exchanger. The resulting gas may then be used to power the piston of a linear actuator which drives the pump. The expansion ratio between the liquid and gas phases is considerable. As the discharge pressure of the pump increases, however, the expansion ratio between the liquid and gas phases becomes less. As a result, the pump becomes less and less effective as its outlet or discharge pressure increases.
Accordingly, it is an object of the present invention to provide a cryogenic fluid delivery system that provides a high discharge pressure with a minimum consumption of utility power.
It is another object of the present invention to provide a cryogenic fluid delivery system that provides a high discharge pressure with an actuating system that is practical and not oversized.
It is still another object of the present invention to provide a cryogenic fluid delivery system that provides for economical saturation on the fly.